My U.S. Pat. No. 5,179,958 and related patents including U.S. Pat. Nos. 5,038,792 and 4,917,708 disclose respiratory calorimeters connected to a mouthpiece which measure the volume of gas inhaled by a patient over a period of time and pass the exhaled gasses through a carbon dioxide scrubber and then a flow meter. Broadly, the integrated flow differences between the inhalations and the carbon dioxide scrubbed exhalations are a measure of the patient's oxygen consumption and thus the patient's metabolic activity. These devices may incorporate a capnometer to measure the carbon dioxide concentration of the exhaled air. A computer receiving signals from the flow meter and the capnometer may calculate, in addition to the oxygen consumption of the patient, the Respiratory Quotient and the Resting Energy Expenditure of the patient as calculated from the Weir equation.
The cardiac output of a patient, that is the volume of blood ejected from the heart per unit time, is another important measured parameter in hospitalized patients. Currently, cardiac output is routinely measured by invasive techniques including thermal dilution using an indwelling pulmonary artery catheter. This technique has several disadvantages including the morbidity and mortality of placing an invasive intracardiac catheter, the infectious disease risks, significant expense and the fact that it provides an intermittent rather than a continuous measurement. A noninvasive, reusable, continuous cardiac output measurement device would substantially improve patient care and reduce hospital costs.
The partial rebreathing technique is a known method for cardiac output measurement. As described in Capek and Roy, "The Noninvasive Measurement of Cardiac Output Using Partial CO.sub.2 Rebreathing", IEEE Transactions on Biomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-659, the method utilizes well known Fick procedures, substituting carbon dioxide for oxygen, and employing a sufficiently short measurement period such that venous carbon dioxide levels and cardiac output can be assumed to remain substantially constant during the measurement. In its original form, the Fick method of measuring cardiac output requires blood gas values for arterial and mixed venous blood as follows: ##EQU1## where C.O. is cardiac output, VO.sub.2 is oxygen consumption, CaO.sub.2 is the arterial oxygen content and CvO.sub.2 is the venous oxygen content. By substituting carbon dioxide for oxygen in the Fick equation, the partial rebreathing method allows computation of cardiac output without invasive blood gas measurements as follows: ##EQU2## The partial rebreathing technique uses the change in CO.sub.2 production (VCO.sub.2) and end-tidal CO.sub.2 in response to a brief change in ventilation. The change in CO.sub.2 production divided by the change in CO.sub.2 content of arterial blood (CaCO.sub.2), as estimated from end-tidal CO.sub.2, equals pulmonary capillary blood flow as follows: ##EQU3## Clinical studies have verified the accuracy of this partial rebreathing method relative to more conventional invasive techniques. Despite the advantages of the partial rebreathing technique it has not achieved extensive usage.
I have discovered that minor modifications of my respiratory calorimeter will enable it to practice cardiac output measurement using the partial carbon dioxide rebreathing technique as well as making the metabolic related measurements described in my patent.